The invention relates to the field of force balancing transducers such as accelerometers and more particularly to the structure of force balancing assemblies used within force sensing instruments.
In prior art instruments, an example of which is the type of accelerometer disclosed in Jacobs, U.S. Pat. No. 3,702,073, assigned to the assignee of this application, the proof mass and force balancing assembly includes a support ring between an upper and lower magnet assembly of the accelerometer and a seismic element, that includes a force restoring coil and bobbin assembly and pick-off capacitor plates, connected by means of one or more flexure-type hinges to the support ring. In this particular instrument the proof mass assembly including the support ring and flexures are configured out of a unitary piece of fused quartz.
Servoed angular accelerometers, servoed pressure transducers, and instruments using search coils, sensing magnetic fields, are other typical examples of instruments using force balancing assemblies.
One of the objectives in designing force balancing assemblies such as the proof mass and force coil assembly shown in U.S. Pat. No. 3,702,073 is to minimize the effect of stress in flexure elements, which in that device connect a seismic element to a support ring, from stress sources, including stresses resulting from a force coil mounting, that can result in strain in the flexures. The strain in the flexures can result in significant bias errors in a servoed instrument. In this servoed accelerometer the pick-off means includes elements on the seismic element force balancing assembly which are used to produce a signal indicating the position of the assembly within the instrument that in turn is used to generate a current in the force balancing coil to restore the seismic element to a zero acceleration position within the instrument. Strain within the seismic element can produce a position signal error. The servo will attempt to zero the position signal error generated, by moving the seismic element, producing a stress or a strain in the flexures in the process. The resulting stress in the flexures produces a moment force which the current in the force coil must balance. The current thus produced in the force coil represents an undesired bias in the current output signal.
As a practical matter a stress free mounting of the force coil on the force balancing assembly is usually not achievable, especially where the force sensing element is made out of a material such as quartz. Quartz has a very low temperature coefficient of expansion compared to that of a force coil which normally is composed of insulated copper wire. Also the adhesive materials used for attaching a coil or bobbin to the force sensing element typically have high temperature coefficients as compared to the materials typically used for force sensing elements of which quartz is one example. A bobbin to form an assembly on which the coil is wound is sometimes used, but does not eliminate the effect of differential temperature expansion even with a match of bobbin and mounting surface temperature coefficients which in any case is usually not practical for other design or fabrication considerations. As a result, there will almost always be some temperature induced stress and strain in the force sensing element through some temperature range which in turn can result in undesired bias errors in the instrument as described above.